Background: The principal determinants of oxygen uptake (V̇O 2) kinetics are controversial, with dynamic changes in central and peripheral factors mediating oxygen supply and utilisation suggested to be limiting. The aim of this study was to determine whether important parameters of blood rheology were related to the exercise-induced time-course changes in V̇O 2 and cardiac output (Q̇c), or steady-state arteriovenous oxygen difference (a-vO 2D) during submaximal cycling. Methods and Results: Blood was collected from ten healthy, recreationally active males and females (age: 21.7 ± 1.3 yr; body mass index: 22.7 ± 2.0 kg·m -2), before each subject cycled at 105% of the first ventilatory threshold. Red blood cell aggregation was negatively correlated with steady-state V̇O 2 during exercise and the a-vO 2D at rest (r = -0.73, p < 0.05), and positively correlated to Q̇c at rest (r = 0.71, p < 0.05). Blood viscosity at various shear rates was negatively correlated with the time constant of V̇O 2 (all p < 0.01) on-transient kinetics. Red blood cell deformability at various shear stress was positively correlated to the time constant of V̇O 2 (all p < 0.05) on-transient kinetics. Conclusions: The findings of the present study suggest that the rheological properties of blood may modulate, at least in part, the rate of change in the uptake and/or utilisation of oxygen at the onset of exercise.